In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC (alternate current) motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and the DC (direct current) voltage of the DC power source is converted into AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in the hybrid automobile or the electric automobile like this, there is known a control device of performing a feedback control on the basis of current sensed values of three phases, which are sensed by the current sensors. For example, a d-axis current and a q-axis current acquired by dq transforming the current sensed values of three phases are fed back to current commands.
Here, there is known the following technique (for example, patent document 1): one current sensor is provided for each phase of a three-phase AC motor; according to the Kirchhoff's law, the sum of currents of three phases to be supplied to the AC motor becomes zero; so that in the case where the sum of current sensed values of three phases does not become zero, it is determined that any one of current sensors is abnormal.
A technique for determining whether or not any one of current sensors is abnormal by monitoring the sum of sensed values of three phases will be described with reference to FIGS. 3A, 3B, and 3C.
At the time of a normal control shown in FIG. 3A, the sum of a U-phase current iu, a V-phase current iv, and a W-phase current iw is always zero, as shown by an equation (1) based on the Kirchhoff's law.iu+iv+iw=0  (1)
Here, for example, as shown in FIG. 3B, it is assumed that an abnormality is caused in which a V-phase current becomes ivs which is offset from iv to a plus side by an error Δis. Hereinafter, this abnormality is referred to as “offset abnormality”. When the offset abnormality is caused, the sum of the current sensed values of three phases (hereinafter referred to as “three-phase sum”, as required) becomes Δis as shown by an equation (2.1), whereby it is found that at least one current sensor is abnormal, that is, “an abnormality of current sensor system” is caused.iu+ivs+iw=(iu+iv+iw)+Δis=Δis  (2.1)
Further, even when a gain error of a variation in current amplitude as well as the offset error caused by the offset abnormality is caused, the three-phase sum does not become zero, so that an abnormality of the current sensor can be detected.
As shown by the following equations (2.2)˜(2.4), however, the detection result (equation (2.3)) in the case where it is assumed that an offset abnormality is caused in the V phase and the detection result (equation (2.2), (2.4)) in the case where it is assumed that an offset abnormality is caused in the U phase or the W phase are absolutely identical with each other and hence cannot be differentiated from each other. In other words, even when the error Δis is caused in any one of phases, the three-phase sum similarly becomes Δis, which hence makes it impossible to identify a phase in which the current sensor is abnormal.(iu+Δis)+iv+iw=Δis  (2.2)iu+(iv+Δis)+iw=Δis  (2.3)iu+iv+(iw+Δis)=Δis  (2.4)
Next, the necessity of identifying a phase in which the current sensor is abnormal will be described.
Firstly, from the viewpoint of trouble shooting, it is necessary to accurately find out an abnormal point. In the case where a current sensor that can be alone replaced is used, only a current sensor made abnormal can be selectively replaced.
Secondly, in the case where only one current sensor is abnormal, by employing the technique of driving the motor by the use of two normal current sensors, the drive of the AC motor can be provisionally continued under a specified condition.
In this way, there is a large need for identifying a phase in which the current sensor is abnormal. As described above, however, it is not possible to identify a phase in which the current sensor is abnormal by the determination using the three-phase sum.    [Patent document 1] Japanese Unexamined Patent Application Publication No. Hei-6-253585